1. Field of the Invention
The present invention relates to the field of physical and chemical analysis of petroleum samples. What is referred to as a “petroleum sample” is a hydrocarbon mixture, that is a mixture of organic compounds mainly containing carbon (C) and hydrogen (H) atoms.
In particular, the invention provides a method of determining physico-chemical properties of a petroleum sample from a quantitative analysis of the molecular composition thereof.
2. Description of the Prior Art
The manufacture of commercial fuels, the optimization and the manufacturing control of the various bases of gasoline pools (gasoline, diesel fuel or kerosene) require precise control of macroscopic physical properties in order to meet market specifications. Macroscopic properties or overall properties are the properties of a petroleum sample that are observed at the scale of the sample, as opposed to the properties that are observed at a smaller scale (molecular compound scale). The macroscopic physical properties are, for example, the octane number, the cetane number, the gravity, etc. Refiners do not have systematically access to the measurement of these physical properties, notably when the amount of fuel available is not sufficient for measurement. Monitoring and optimization of industrial plants producing gasoline pool bases also increasingly requires precise knowledge of the mechanisms involved in the reactors.
Besides, refiners want to predict the impact of a transformation of fuels on the physical properties thereof. For example, it is desired to be able to simulate the effect of a change in the distillation range of a sample, or the effect of the chemical transformation of a family of compounds (hydrogenation of aromatic compounds to saturated cyclic compounds) on properties such as the octane number, the cetane number or the gravity.
In this perspective, knowledge of the macroscopic physical properties of fuels and knowledge of the properties by cuts becomes essential. A cut of a petroleum sample is the distillate obtained between two predetermined temperatures during a distillation operation. Refiners' needs are increasingly directed towards the development of explicit property models as regards the chemical composition of fuels.
Physical methods have been standardized to assess these properties, but they are generally costly in time and they require a large volume of samples for implementing them.
For samples distillable between 0° C. and 220° C. (gasoline and naphtha cuts), these physical methods have been advantageously replaced by the use of cut mixing laws based on the refiners' experience, or by the use of dedicated analytical tools allowing the composition of the products to be related to a physical property. Modern chemical analysis methods such as infrared spectrophotometry (ISP) or gas chromatography (GC) have thus allowed access to more complete molecular information on gasoline cuts, and the development of correlations between the chemical composition of the gasoline obtained by these analysis techniques and several properties such as the octane number, the gravity, the calorific value, etc. The first research work was carried out from gas chromatography (GC) analysis and it is described in the following document: Jenkins G. J., Mc Taggart N. G., Watkin B. L. H., (1968) “GLC for On-Stream Octane Number Rating of Stabilized Catalytic Reformates”, Gas Chromatography, Ed. Inst. Petr., London, p 185-198.
Models obtained by linear regression were elaborated. They group together the chromatographic data (mass concentration of the individual constituents (compounds)) into about thirty groups of constituents according to chemical structure analogy. In the early 80s, the first publications relative to the calculation of octane numbers from near infrared spectrometry appeared: Kelly J. J., Barlow C. H., Jinguji T. M. and Callis J. B. (1989) “Prediction of Octane Numbers from Near-Infrared Spectral Features in the Range 660-1212 nm”, Analytical Chemistry, Vol 61, No 4, 313. The work by D. Lambert et al. marked the beginning of the use of this technique as a control tool for the gasoline pool of refineries: Espinosa A., Lambert D. and Valleur M. (1995) “Use NIR technology to optimize plant operation, Hydrocarbon Processing”, February, 86. According to the same approach, J. P. Durand, Y. Boscher, N. Petroff and M. Berthelin, J. Chromatogr., 395 (1987) 229, showed that it is possible to describe the density, the gravity, the Reid vapour pressure, etc., from a gasoline compositional analysis.
For samples distillable between 220° C. and 450° C. (case of diesel fuel cuts and middle distillates), other techniques had to be developed. In fact, such a compositional detail is not available for diesel fuel cuts or middle distillates that involve severe limitations regarding the aforementioned analysis methods. It is known that the increase in the number of isomers with the number of carbon atoms makes the extension of this degree of information to heavier cuts than gasoline illusory. For GC, the lack of separation power does not allow having a distribution of the hydrocarbons by families. This makes the property prediction models based on a molecular description ineffective. Only the mass distribution of the components as a function of the boiling point is accessible. As for mass spectrometry, only more summary molecular information is available (obtained according to the method referred to as Fitgerald's method (reference number ASTMD2425)). However, this method is applicable only to samples having a well-established range of distillation intervals (set initial and end points, fraction distillable by at least 70° C. in the interval), or limited olefin contents (2% m/m maximum). Besides, it does not allow separate quantification of the linear and branched paraffins, which does not enable determination of some properties that greatly depend on the branching rate (cetane number, gravity, etc.).
Specialists then combine the data obtained by GC, SM or any other informative detector (such as the atomic emission detector AED) to bypass the limitations intrinsic to each one of these techniques and predict the macroscopic properties of the middle distillate or kerosene cuts.
U.S. Pat. No. 5,699,269 describes a method allowing prediction of the macroscopic properties of petroleum cuts from a gas chromatography analysis coupled with mass spectrometry (GC/MS). This method presupposes that the property model calibration basis contains samples whose composition is close to that of the sample to be analyzed. This invention allows prediction of the macroscopic properties of petroleum cuts but it is not applicable to the prediction of cut simulation or chemical transformation simulation properties. In other words, this method is not extrapolatable outside its calibration basis. It does not meet the need for fuel property simulation because it is not based on the properties-fine chemical composition of the fuels link.
U.S. Pat. No. 6,275,775 describes a method of predicting the properties of petroleum fractions by means of gas chromatography coupled to an atomic emission detector (GC-AED). The properties predicted are macroscopic but they also depend on the distillation range of the sample. This invention thus allows deducing from the GC-AED analysis the property profiles of diesel fuels as a function of the distillation curve. On the other hand, it does not allow simulation of a chemical transformation of a petroleum cut (for example the conversion of aromatic compounds to saturated cyclic compounds by a hydrotreatment method).
The approach using near infrared spectrophotometry was also implemented on middle distillates. However, this method is correlative and it greatly depends on the representativity of the database. Furthermore, it is not applicable to the prediction of cut simulation or chemical transformation simulation properties. In other words, this method is not extrapolatable outside its calibration basis. It does not meet the need for fuel property simulation because it is not based on the properties-fine chemical composition of the fuels link.
In conclusion, the known methods are based on correlative models and not on explicative models insofar as they are not based on the molecular detail of the petroleum fractions being analyzed. In particular, the conventional analytical methods for analyzing samples distillable between 150° C. and 450° C., such as diesel fuel cuts, do not provide sufficient analytical detail to allow modelling of the application properties of these samples. The key point for the prediction of properties lies in the close relationship between the property to be predicted and the detailed chemical composition (analysis by families and by number of carbon atoms for example for petroleum products).